1. Technical Field
The present invention relates to a semiconductor integrated circuit, and more particularly, to a semiconductor integrated circuit having a variable resistive element.
2. Related Art
Magnetic random access memory (MRAM) is a memory device configured using a magnetic tunnel junction (MTJ) memory element. MTJ is a structure in which two ferromagnetic layers formed at both sides of a middle insulating layer. In the MTJ, the ferromagnetic layer formed at one side of the insulating layer is referred to as a pinned layer or fixed layer of which spin direction is not changed by an external stimulus, and the ferromagnetic layer formed at the other side of the insulating layer is referred to a free layer of which spin direction is relatively easily changed by an external stimulus such as an external magnetic field or spin transition of through current. A thin insulating layer for electrical/magnetic insulation such as MgO layer, according to an example, having a thickness of 1 nm or less, is formed between the two ferromagnetic layers.
The resistance of the MTJ is changed depending on a magnetic orientation state of the two ferromagnetic layers that constitute the MTJ, which is referred to as a tunneling magneto resistance (TMR) phenomenon. The ratio between two resistances depending on the magnetic orientation state of the two ferromagnetic layers is referred to as a TMR ratio. In the MRAM, data is recorded or read using a resistance change of the MTJ, i.e., a variable resistance.
A field induced magnetic switching (FIMS) method and a spin transfer torque (STT) method are used as the recording method of the MRAM. The FIMS method is a method of changing the spin direction of the free layer using a strong external magnetic field. However, since the FIMS method requires a separate component for generating an external electric field, it is not easy to perform the integration of the MRAM.
On the other hand, the STT method is a method of changing the spin direction of the free layer through spin transition of electrons by orienting the spin direction of the electrons that pass through the MTJ using the influence of interaction/reaction generated between the spin characteristic of the electrons and the spin direction of the ferromagnetic layer when the electrons pass through the ferromagnetic layer and then passing the electrons through the free layer. That is, the STT method adjusts the variable resistance of the MTJ by controlling the spin direction of the MTJ according to the direction of current passing through the MTJ.
As described above, the variable resistance of the MTJ using the STT method is adjusted by the current passing through the MTJ.
In the adjustment of the variable resistance of the MTJ, an amplitude of a current pulse increases as the width of the current pulse flowing through the MTJ decreases. Here, the width of the current pulse may be a duration of the current applied to the MTJ. However, if the amplitude of the current pulse increases, the voltage applied to both ends of the MgO that is an insulating layer of the MTJ increases to increase the amplitude of the current pulse, and thus the element characteristic of the MgO may be degraded. The degradation of the insulating layer may result in the resistance drift phenomenon of the MTJ and reduce the reliability of the element.
On the contrary, if the width of the current pulse flowing through the MTJ increases to decrease the voltage applied to both the ends of the MgO, the degradation of the insulating layer may also be caused. If the duration of the current flows in the MgO increases, the MgO may be degraded, and therefore, the TMR ratio that is a main index of variable resistance characteristics of the MTJ may deteriorate. Accordingly, the lifespan of the MTJ may decrease.
The characteristics of the MTJ have influence on the timing of a recording operation of an STTMRAM. In order to perform a stable recording operation of the STTMRAM, a width of a writing current pulse, i.e., current flowing through the MTJ is necessarily set to have a sufficient margin. However, an operation speed of a semiconductor memory device may deteriorate due to the setting of the sufficient margin to the width of the current pulse flowing through MTJ.